The intent of this proposal is to study the local circuitry in the inferior olive (IO) of the rat using extracellular and intracellular injections of horseradish peroxidase (HRP). Data obtained from physiological studies on the olivo-cerebellar system have postulated that the output of the IO, which exerts a powerful excitatory effect on Purkinje cells in the cerebellar cortex is patterned by intrinsic membrane properties of IO neurons, as well as by local excitatory and inhibitory circuits. It is hypothesized that the local circuits are derived from axon collaterals of IO neurons or from neurons located in the adjacent peri-olivary reticular formation (PRF). The specific aims of this study are to determine: 1) the location of neurons in the PRF which extend their processes into the olivary neuropil; 2) the morphological characteristics of the PRF neurons which project to the IO; 3) the type and distribution of IO neurons that give rise to axon collaterals which arborize in the caudal brainstem; 4) the brainstem distribution of the axon collaterals of olivary neurons; and 5) the ultrastructural features of the local interactions formed by neurons in the IO and PRF. Data from these studies will provide the anatomical substrate for the local interactions postulated to exist in physiological studies. In addition they will be used in future physiological studies on PRF and IO neurons. These type of data will extend our knowledge as to how the IO controls movement through its input to the cerebellum.